FORMATION OF VEINS. 715 



be such as to produce a galvanic effect, further promoting decom- 

 positions and recompositions. When the solutions differ, after 

 intervals of time there will be a succession in the changes, and 

 layers of different species may be formed. 



Thus, a layer of quartz may be succeeded by one of fluor spar, or of zinc 

 blende, or of calcite, or of quartz again, etc. In the course of the changes, a 

 layer of cubes of fluor spar, underlying one of quartz, may be entirely dissolved 

 away, and the cubical cavities filled up by another species, as zinc blende, etc. 



The rock of the walls (especially of the lower wall, where the vein is inclined), 

 when not united firmly to the vein, often undergoes deep alteration, and may 

 become penetrated by ores from the vein itself, carried in by infiltrating solu- 

 tions. These alterations are most extensive in the upper part of veins, where it 

 often happens that the metals are removed by infiltrating waters, excepting for 

 the most part the iron, which is left in the state of red oxyd, giving its color to 

 the earthy mass at the top of the vein (called then the iron hat). Hence the 

 occurrence of a line of red earth in the soil may be an indication of a vein of 

 ore beneath. 



Gold-bearing quartz veins generally lose the pyrites and perhaps other ores 

 which they contain, and thus become cavernous to a considerable depth. To 

 this distance they are mined with comparative ease ; but beyond they are ex- 

 tremely hard and difficult to work. 



4. Veins of different ages. — In the progress of the uplifting and 

 folding of a region undergoing metamorphism, fissures formed at 

 one time and filled would be liable to be broken by cross-fissures 

 at some subsequent time in the epoch (perhaps a following day, 

 week, or year), and these, again, by others. Thus, a succession of 

 veins faulting one another might be formed during one epoch of 

 disturbance, and they might differ in constitution as the bands in 

 a banded vein differ. 



Again, veins may be intersected and faulted by fissures formed 

 during subsequent epochs of disturbance. 



It is evident, therefore, that a vein which faults another does not 

 necessarily belong to a later independent epoch. When actually 

 later in epoch, it will usually appear in the distribution of the new 

 veins over a wide region of country, their general parallelism of 

 direction, and their wholly distinct mineral composition. 



5. Filling of amygdaloidal cavities. — The cavities of a lava or igneous rock 

 (such as are formed by expanding vapors while the rock is liquid) differ from 

 veins in size, but not essentially in the method by which they are filled with 

 minerals. In amygdaloids, these minerals are usually chlorite, quartz, prehnite, 

 datholite, analcime, or some of the zeolites, or calcite ; and they often occur in 

 successive layers, analogous to the layers of a banded vein. They are intro- 

 duced by infiltrating waters which derive the ingredients mainly from the en- 

 closing rock through the decomposition of some of its minerals. Quartz (glassy 

 quartz, chalcedony, agate, carnelian, etc.) and calcite are the most common of 



